Light source feed-back system



W. R. LE PAGE LIGHT SOURCE FEEDBACK SYSTEM Filed April 50, 1945 Nov. 19,1946.

Patented Nov. 19, 1946 LIGHT- SOURCE FEED -BACK- SYSTEM Wilbur labile,lPage, Pikesville, :Ma., assignonto, fjtadio Corporation of America, yacorporation of Delaware iApplicationiApril'i-bfl, 1943, Serial No.485,255

"5 ,Claims.

This invention relates to photographic sound .recording and reproducingsystems, and, partic u- .larly to ,a system ,-for stabilizing the 1light, sources .used. in suchsystems or anysystem iniwhich the lightoutput should varydinearly with anapplied .signal, :voltage ,or currentwhich :may consist of oscillations ,of :audio frequencies andunidirectional-components. i

It ,is W811 known :that. in, photographic "sound erecording.systemsdightsources orlampsare-em- .ployed ,toproduce lightibeamswhichmay. be eitherdirectly,mod.ulated ,by varying the f energy supplied,togthelampsiinaaccordance with ,the variationslof soundwavestoberecorded, or whichtmay be modulated by vibrating the beaminaccordancewith thesoundwaves. Light. sources cfconstant .intensity are .usedi'for,recording both variable ,area-and variable density sound tracks, thelight being modulated by the vibrations :of agalvan- ;ometer mirror."Light sourceaisuch as glowlamps, which vary in intensity are alsoemployed for recording variable density records. For reproducing eithertype of record, *a light source of constant intensity is used to producethe scanning'beam. I

When alight of constantintensity is employed 'for'recording; it isnecessary that it be maintained "accurately constantly to preventundesiredvariati'ons inthe sound levels or modulations and theintroduction of distortioninthe recorded or re- 'produced sound."Unwanted light variations may gizing source, by deterioration 'of thelight-producing elements, bythe blackening ofthe lamp envelope 'andother reasons. When the recordingis done'by causing a variation of thelight source, "this variation must be linear with the appliedsignaL-voltage or current, and the linearity must be maintained forfrequencies up to the highest frequency of sound to be recorded-Whilethe'average light remains constant or at a controllable value. Thatis,the light'must follow an averagevaluewhich-is determined by the noisereduction'voltage and'also thesoundsignal frequencies andthus linearlyrespond to voltage or current of-anyffrequencyin theband tob'e-recordedand adirect-currentor voltage. 'Theinventi-on further provides-afeedback system which --requiresveryJlittlelight toproducethe necessarystabilization, modulation,:and control.

"The present inventiongtherefore, is directed to a-systemforstabilizingand providing linearity of light variation 1 with "an appliedalternating and ;direct .;current signal. This is accomplishedjby.detecting a portionxofftheflight;andrregulatingihe (Cl. 250-415) g 2energy Vsuppliedit'o Ethe :lamp in accordancewvith "the variationsinzthe ,deteotedgllight for-stabiliza- .ztiongand in accordancewithsanapplied signal for modulationcand ground noise reduction. ziThe 5general principle ,of suchan opticalieedback stabilizing systemkisknownrin the::art. Incertain .of "these IDIiOI' systems; which provide:both direct current, andalternating-current feedback, the impedance ofanqelectronicsrdevice connected in series with the :lamp :and ithe"primary? power :source'is varied inaccordance .with the:light fluc-:tuations. i-The amountx'ofvfeedback in suchra system "depends ontheigain .Lof tth'e amplifiergthe eiiiciency of translationof .the:light 'JSEYISltlVSCdG- vice and the amount of ilightgcollected; foracontrol purposes. For a fixed amount-otzamplification and I afixedramount :of; lightcreceived: by; the 'photocell, :thefeedback can:only ;bex-.increased :by increasin'g the :rcell xload resistance, :orfor i a 20 1 fixed-number of #stages ,of; amplificatiomthe'feedbackmaybe increased by; increasingthe,interstage coupling=resistors. ;However, these methods of increasing ifeedback have thedisadvantage :;of causing :phase shift ,and ilossiof gain :at ;,highifrequencies. i l

I'he zprincipalgobiectaoflthe invention; therefore, ,is'tor-improvelight ;source stabilizing, modulating, and controlling systems.

:Another; obj ect.ofxgthezinvention is: torprovide an opticalfeedback;stabilizing;modulating; and con- ;trolling "system which rhasIminimum distortion and axhighisensitivityrto the controlling light,making it: unnecessary to appreciably reduce theuseful,light,:tmObtaimSuflicient control light.

:A'furt-herobject ofttheiinvention is to-provide :responding tocrapi'dwchanges innlight ioutput, was well .asgto slow ,vari-ationsztherein. 1

:Althoughtthe; novelfeatures which are believed tot-be characteristic,Of'ilhisf invention are pointed ;out with {particularity-1m 1511116?claims "appended rhereta-gfthe manner :jof 1 its rorganization :and theimodeloif iits ,,operation'vvill'cbebetter understood zbyzreferring" to:the following :description, :read in conjunction withrtheiaccompanyinggdrawing, inwhich: r i r {Fig :1 iisra ,rbiock :diagra-mshowing :the ":general 1 arrangementofg thezsystenrembodyine the-inven-:tion., V

.Eig. ..-2 is ca schema-tic 5 dia amof thefieedback amolifienofthei-invention-nand 1 Fig.3 is. a schemat-imdetail ,;diagram; ofarmodimfication of:.the,glastqstagezofuthe system shownmof YEigJQZ. c 55:Reierringmow to EEiaLl; blockimisranamplifler :an optical feedbacksystem "which is capable ;of

4 sistors 26, 21, and 28. The effective plate loading resistance oftubes l5 and I6 is produced by resistor 26 and resistor 30 in parallelfor tube l5 and by resistors 21 and 34 in parallel for tube IS. Theconstant voltage drops across gas-filled voltage regulator tubes 32 and35 produce the proper biaseson the tubes l5 and 16, respectively,

. and effectively provide a'direct coupling between conductors l0 uponthe input of amplifier A in series with an external voltage which may beconsidered as co, the total voltage thus appearing across the input toamplifier A being e1. The phase of the voltage ez is such'as'todecreasethe energizing lamp current with an increase in light output. 7

The entire system just described can be considered as a feedbackamplifier with 100 per cent feedback. Without feedback, the gain G ofthe I amplifier may be considered as 62/61. It is evident that ez isconsidered the output voltage, in order to deal with the circuit as anamplifier. Actually, the light is the useful output, but since thelightis proportional to e; the discussion is valid. Thus, in the usualnotation in which the feedbackfact'or 5 equals (1), the'well knownformula G/(1.GB)=G/(I+G) .for the'gain of a' feedback amplifier appliesin this case. If G is large,the gain'is nearly unity, and this is thecondition desired. In this analysis the numerical value of the overallgain is based on the assumption that 13:1. tion of operation is thatthevalue of G be sufli- Now, the optimum condiciently large so thatthe netgain is very nearly 7 equal to unity.

' of'feedback is limited by the size of the photocell and amplifierload'resistors and any increase therein increases the effects of straycapacity with the associated difiiculties. Of particular concern in thisapplication is the resulting phase shift at the higher frequencies,which may contribute to a tendency to'osoillate. This phase shiftdisadvantage may be more acute with the use of a glow lamp since thelight modulation lags the current modulation by a considerable angle athigh audio frequencies' The present invention overcomes the abovedifficulties by the useof morestages of amplification'in combinationwith low coupling resistances and a low photocell load resistance.Referring now to Fig.2, four stages l5, l6, l1, and I8, of amplificationare shown,; the first three tubes l5, l6, and I1 being comparable to"amplifier A of Fig. 1, and tube l8 being comparable to power unit P ofFig. 1. The three tubes l5, l6, and I! may be amplifier tubes, such asthe RCA type 6J7,"while tube IBmay be of the RCA type 6L6. The lamp 5 ofFig. 1 is shown connected in the plate circuit of tube I 8, energyforthe lamp and tube being supplied by a battery 20. The photoelectriccell 6 of Fig. 1 is shown connected back .to a low value load'resistor22in the input of tube l5.

The external voltage represented by E6 in Fig. 1

24, one of which may be usedfor'introducing a maybe impressed acrosseither resistors 23 or modulating signal voltage and the other forintroducinga noise reduction voltage.' A potentiometer 42 is anintensity control for the lamp S'which may'also be obtained, if desired,by a =volta'ge applied across either resistor 23 or resistor 24. Acommon source of B potential is suppli ed'to the 'plat'sof tubes l5, l6,and l 1 over re- 4i regulates the bias potential on tube 18.

Since a sustaining current of about 5 mils is necessary in the glowtube, thecoupling resistors cannot be very large, and this isparticularly desirable in order to provide the low coupling between theamplifying stages. The above -mentioned circuit operated satisfactorilyto provide a fiat frequency response characteristic up to 10,000 cyclesper'second with negligible distortion. To obtain 20 db. of feedback withresistor-"22 having the small value of 10,000 ohms, only one microampereof photocell current is necessary for each ten milliamperes of lampcurrent. More photocell current, (i. e. more detected light) will givemore feedback. The system may thus supply a considerable amount offeedback with a very feeble photocell current, and therefore has a highsensitivity to detected light. The modulating signal and noise reductionvoltages may also be introduced in the control grid circuit offtube [5instead of across resistors. .23 and 24 as mentioned above.

In Fig. 3, an alternate method of connecting the- 6L6 into the circuitis shown, a method in which the screen grid of the tube is separatelyutilized. The same low coupling resistances betweentub es is obtainedwherein the amount of feedback'fis ,more nearly constant as a functionof frequency.

I claim as my invention: a i 1. A feedback amplifier system forstabilizing a light source comprising a photocell, apotential source forsaid photocell, an input stage of amplification having its inputconnected tosaid photocell, a light source to be controlled, an .outputstage of amplification having its output connected to said light source,coupling means between said amplifying stages, said coupling meanshaving a low value of resistance, and coupling means intermediate saidphotocell and said input stage of amplification, said last mentionedcoupling. means including .a resistor .in; series With said photocellandlsaid potential source, said resistor having a resistance value ofthe .order of 10,000 ohms, said low value of resistance minimizing phaseshift and loss of gain-at high frequencies. a

2. A' feedback-amplifier system for stabilizing a light sourcecomprising a photocell, an input stage of amplification having its inputcircuit connected to said photocell, a coupling resistor in'said inputcircuit, a light source to be controlled, an output stage ofamplification having its output connected to said light soure -couplingmeans between said amplifying stages-said coupling means having a lowvalue of resistance, a polarizing source of voltage for said'photocelland the anodes'of the tubes in said amplifying stages, and adegenerative feedback series-circuit from said polarizing source ofvoltage sistor and back to said polarizing sourcefor coupling saidphotocell to said input stage of amplification.

3. A feedback amplifier system for stabilizing a light source comprisinga photocell, an input stage of amplification having its input circuitconnected to said photocell, a coupling resistor in said input circuit,a light source to be controlled, an output stage of amplification havingits output connected to said light source, coupling means between saidamplifying stages, said coupling means having a low value of resistance,a polarizing source of voltage for said photocell and the anodes of thetubes in said amplifying stages, and a series circuit from saidpolarizing source of voltage through said photocell through saidcoupling resistor and back to said polarizing source for coupling saidphotocell to said input stage of amplification, the value of saidcoupling resistor being of the order of 10,000 ohms.

4. A feedback amplifier system for stabilizing a light source comprisinga light source to be stabilized, a photoelectric cell for detecting aportion or the light from said source, a plural stage amplifierincluding at least three stages intermediate said photocell and saidlight source, glow lamp coupling means between said amplifying stages, ahigh potential source for said photocell and the anodes of the tubes insaid amplifier stages, said coupling means having a low value ofresistance, and coupling means between said photocell and said firststage of amplification, said last-mentioned coupling means in cluding aresistor in series with said high potential source and said photocell toprovide a degenerative ieedback circuit, the variations in voltageacross said resistor being amplified by said amplifier stages.

5. A feedback amplifier system for stabilizing a light source comprisinga light source to be stabilized, a photoelectric cell for detecting aportion of the light from said source, a plural stage amplifierincluding at least three stages intermediate said photocell and saidlight source, glow lamp coupling means between said amplifying stages, ahigh potential source for said photocell and the anodes of the tubes insaid amplifier stages, said coupling means having a low value ofresistance, and coupling means between said photocell and said firststage of amplification, said last-mentioned coupling means including aresistor in series with said high potential source WILBUR R. LE PAGE.

